This invention relates generally to fluid processing, and specifically to process flow measurement and control.
Magnetic flowmeters (or magmeters) measure flow by Faraday induction, an electromagnetic effect. The magnetic flowmeter typically includes a flowtube and a transmitter. The flowtube includes a pipe, a field coil (or coils) mounted on the pipe, and electrodes that extend through the pipe. The transmitter energizes the field coil (or coils) to generate a magnetic field across a pipe section, and the magnetic field induces an electromotive force (EMF) across the process flow. The resulting potential difference (or voltage) is sensed using a pair of electrodes that extend through the pipe section and into contact with the process flow, or via capacitive coupling. The flow velocity is proportional to the induced EMF, and the volumetric flow rate is proportional to the flow velocity and flow cross-sectional area. The transmitter receives the sensed voltage from the electrodes and produces a signal representing measured flow.
In general, electromagnetic flow measurement techniques are applicable to water-based fluids, ionic solutions and other conducting flows. Specific uses include water treatment facilities, high-purity pharmaceutical manufacturing, hygienic food and beverage production, and chemical processing, including hazardous and corrosive process flows. Magnetic flowmeters are also employed in the hydrocarbon fuel industry, including hydraulic fracturing techniques utilizing abrasive and corrosive slurries, and in other hydrocarbon extraction and processing methods.
Magnetic flowmeters provide fast, accurate flow measurements in applications where differential pressure-based techniques are disfavored because of the associated permanent pressure loss (for example, across an orifice plate or Venturi tube). Magnetic flowmeters can also be used when it is difficult or impractical to introduce a mechanical element into the process flow, such as turbine rotor, vortex-shedding element or Pitot tube.
Some magnetic flowmeters use field coils driven directly by AC line power. Another type of magnetic flowmeter, commonly referred to as a pulsed DC magnetic flowmeter, excites or powers the field coil periodically with a low frequency square wave. Pulsed DC magnetic flowmeters utilize a magnetic field which changes direction at a certain frequency.
There are certain situations that can cause the power provided to the field coils to exceed the capabilities of the magnetic flowmeter. These can occur at initial startup as a result of a flowtube having different operating characteristics than the transmitter, or during normal operation due to changes in field coil resistance caused by terminal corrosion, or excessive temperature in process conditions and/or field coils.
Some of the symptoms that will be exhibited if the power limit is exceeded include: overheating and damaging the flowtube field coils or the transmitter electronics, or both; potentially dangerous surface temperature of the flowtube due to excess heating; and continual power cycling of the transmitter due to power starvation, which can prevent the user from switching the configuration of the magnetic flowmeter or observing any diagnostic warning messages.
In magnetic flowmeters, the coil current and the number of windings of the field coil(s) determine strength of the magnetic field perpendicular to the conductive process fluid flowing through the flowtube. The flow rate of the process fluid cutting across this magnetic field produces a small potential on the electrodes exposed to the process fluid. The signal produced on the electrodes is directly (linearly) proportional to flow rate for a given number of windings (turns) and the given coil current in the windings.
The circuitry typically used to drive the magnetic field, although stable and controllable in normal operating conditions, does not provide an independent or redundant current limitation in failure modes of the transmitter or the flowtube. A variety of methods have been used to limit current to field coil windings of a flowtube to ensure that the winding insulation is not damaged and does not exceed the thermal class (Class 180, Class 200, etc.) in failure conditions. These methods have employed inline resistive limiting (such as from simple resistors), fuses, or active semiconductors, such as power FETs or SCR's. Drawbacks of typical solutions can include unacceptable power loss, excessive over-temperature requirements, complex safety circuit topology sensitive to external transient conditions, and one-time use due to destructive interrupt.